Field effect transistor

ABSTRACT

A field-effect transistor includes an n-type semiconductor layer that includes a Ga2O3-based single crystal and a plurality of trenches opening on one surface, a gate electrode buried in each of the plurality of trenches, a source electrode connected to a mesa-shaped region between adjacent trenches in the n-type semiconductor layer, and a drain electrode directly or indirectly connected to the n-type semiconductor layer on an opposite side to the source electrode.

TECHNICAL FIELD

The invention relates to a field-effect transistor.

BACKGROUND ART

A Ga₂O₃-based trench MOSFET having a gate electrode buried in asemiconductor layer is known (see, e.g., Patent Literature 1). Ingeneral, trench MOSFETs are characterized by having a loweron-resistance than planar MOSFETs.

According to Patent Literature 1, threshold voltage is increased byusing a p-type β-Ga₂O₃ single crystal film as a semiconductor layer inwhich the gate electrode is buried, as compared to using an undopedβ-Ga₂O₃ single crystal film. For this reason, in order to obtainsufficient off-state leakage characteristics in high current powerdevices, the gate electrode should be buried in the p-type β-Ga₂O₃single crystal film.

CITATION LIST Patent Literature

Patent Literature 1: JP 2016/15503 A

SUMMARY OF INVENTION Technical Problem

However, it is very difficult to produce a p-type β-Ga₂O₃ singlecrystal. Therefore, the Ga₂O₃-based trench MOSFET disclosed in PatentLiterature 1 in which the gate electrode is buried in the p-type β-Ga₂O₃single crystal film is difficult to manufacture.

It is an object of the invention to provide a Ga₂O₃-based field-effecttransistor which is excellent in off-state leakage characteristics andbreakdown voltage without using the p-type β-Ga₂O₃ single crystal.

Solution to Problem

To achieve the above-mentioned object, an aspect of the inventionprovides a field-effect transistor defined by [1] to [7] below.

[1] A field-effect transistor, comprising: an n-type semiconductor layerthat comprises a Ga₂O₃-based single crystal and a plurality of trenchesopening on one surface; a gate electrode buried in each of the pluralityof trenches; a source electrode connected to a mesa-shaped regionbetween adjacent trenches in the n-type semiconductor layer; and a drainelectrode directly or indirectly connected to the n-type semiconductorlayer on an opposite side to the source electrode.

[2] The field-effect transistor according to [1], wherein the gateelectrode comprises a p-type semiconductor, and the gate electrode andthe n-type semiconductor layer are in contact with each other to form ap-n junction.

[3] The field-effect transistor according to [1], wherein the gateelectrode comprises a conductor, and the gate electrode is electricallyinsulated from the n-type semiconductor layer by a gate insulator film.

[4] The field-effect transistor according to [3], further comprising: ap-type semiconductor member connected to at least a part of themesa-shaped region and to the source electrode.

[5] The field-effect transistor according to any one of [1] to [4],wherein an edge portion of the source electrode is located lateral tooutermost one of the gate electrodes.

[6] The field-effect transistor according to any one of [1] to [5],wherein the mesa-shaped region has a width of not less than 0.1 μm andnot more than 2 μm.

[7] The field-effect transistor according to any one of [1] to [6],wherein a distance from a surface of the n-type semiconductor layer on aside of the drain electrode to a bottom of the trenches is not less than1 μm and not more than 500 μm.

Advantageous Effects of Invention

According to the invention, a Ga₂O₃-based field-effect transistor can beprovided which is excellent in off-state leakage characteristics andbreakdown voltage without using the p-type β-Ga₂O₃ single crystal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross-sectional view showing a trench MOSFET (MetalOxide Semiconductor Field Effect Transistor) in the first embodiment.

FIG. 2 is a vertical cross-sectional view showing a modification of thetrench MOSFET in the first embodiment.

FIG. 3 is a vertical cross-sectional view showing another modificationof the trench MOSFET in the first embodiment.

FIG. 4 is a vertical cross-sectional view showing a trench JFET(Junction Field Effect Transistor) in the second embodiment.

FIG. 5 is a vertical cross-sectional view showing a modification of thetrench JFET in the second embodiment.

FIG. 6 is a vertical cross-sectional view showing a trench MOSFET in thethird embodiment.

FIG. 7 is a SEM (Scanning Electron Microscope) observation image showinga cross section of a trench MOSFET in Example 3.

FIG. 8A is a graph showing DC characteristics of the trench MOSFETpertaining to FIG. 7.

FIG. 8B is a graph showing transfer characteristics of the trench MOSFETpertaining to FIG. 7.

DESCRIPTION OF EMBODIMENTS First Embodiment (Configuration of TrenchMOSFET)

FIG. 1 is a vertical cross-sectional view showing a trench MOSFET (MetalOxide Semiconductor Field Effect Transistor) 1 in the first embodiment.The trench MOSFET 1 is a vertical field-effect transistor having atrench-gate structure. The trench MOSFET 1 in the first embodiment alsoincludes a configuration in which gate insulator films 13 (describedlater) are formed of a material other than oxide.

The trench MOSFET 1 is provided with an n-type semiconductor substrate10, an n-type semiconductor layer 11 formed on the n-type semiconductorsubstrate 10 and having trenches 16 opening on the upper surface (asurface opposite to the n-type semiconductor substrate 10), gateelectrodes 12 buried in the trenches 16 of the n-type semiconductorlayer 11 in a state of being covered with gate insulator films 13, asource electrode 14 formed on the upper surface of the n-typesemiconductor layer 11, and a drain electrode 15 formed on a surface ofthe n-type semiconductor substrate 10 on the opposite side to the n-typesemiconductor layer 11.

The trench MOSFET 1 may be either normally-off or normally-on, but isusually manufactured to be normally-off in view of safety when used as apower device. It is to prevent conduction between the source electrode14 and the drain electrode 15 in the event of power outage.

In the normally-off trench MOSFET 1, channels are formed in themesa-shaped regions of the n-type semiconductor layer 11 between theadjacent trenches 16 when voltage of not less than threshold voltage isapplied between the gate electrodes 12 and the source electrode 14,allowing a current to flow from the drain electrode 15 to the sourceelectrode 14.

The n-type semiconductor substrate 10 is formed of an n-type Ga₂O₃-basedsingle crystal containing a group IV element such as Si or Sn as adonor. The donor concentration in the n-type semiconductor substrate 10is, e.g., not less than 1.0×10¹⁸ cm⁻³ and not more than 1.0×10²⁰ cm⁻³.The thickness of the n-type semiconductor substrate 10 is, e.g., notless than 10 μm and not more than 600 μm.

The Ga₂O₃-based single crystal here means a Ga₂O₃ single crystal or is aGa₂O₃ single crystal doped with an element such as Al or In, and may be,e.g., a (Ga_(x)Al_(y)In_((1-x-y)))₂O₃ (0<x≤1, 0≤y<1, 0<x+y≤1) singlecrystal which is a Ga₂O₃ single crystal doped with Al and In. The bandgap is widened by adding Al and is narrowed by adding In. The Ga₂O₃single crystal mentioned above has, e.g., a β-crystal structure.

The plane orientation of the n-type semiconductor substrate 10 is notspecifically limited, but is preferably a (001) plane on which aGa₂O₃-based single crystal constituting the n-type semiconductor layer11 is formed at a high growth rate.

The n-type semiconductor layer 11 is formed of an n-type Ga₂O₃-basedsingle crystal containing a group IV element such as Si or Sn as adonor.

The n-type semiconductor layer 11 has a channel layer 11 b in which thegate electrodes 12 are buried and channels are formed when gate voltageis applied, a drift layer 11 a provided under the channel layer 11 b tomaintain breakdown voltage, and a contact layer 11 c formed in thevicinity of the upper surface of the n-type semiconductor layer 11 byion implantation or epitaxial growth, etc., to provide an ohmicconnection between the source electrode 14 and the n-type semiconductorlayer 11.

A distance D from a surface of the n-type semiconductor layer 11 on thedrain electrode 15 side (an interface between the n-type semiconductorsubstrate 10 and the n-type semiconductor layer 11) to the bottoms ofthe trenches 16 is one of parameters determining the breakdowncharacteristics of the trench MOSFET 1, and given that breakdown fieldstrength of Ga₂O₃ stays constant at 8 MV/cm which is a value estimatedfrom the band gap, the distance D needs to be, e.g., at least not lessthan about 1 to 2 μm to obtain performance of having breakdown voltageof 600V used for home appliances or in-vehicle devices, etc., not lessthan about 3 μm to obtain breakdown voltage of 1200V used for industrialequipment, etc., not less than about 8 to 9 μm to obtain breakdownvoltage of 3300V used for large transportation equipment such as bullettrain, etc., not less than about 16 to 17 μm to obtain breakdown voltageof 6600V in high power applications such as power generation andtransmission, etc., not less than about 30 μm to obtain breakdownvoltage of 12000V in medium-voltage circuit breakers, and not less thanabout 250 μm to obtain breakdown voltage of 100000V in high-voltagecircuit breakers. The maximum breakdown field strength of Ga₂O₃ has notbeen able to be actually measured at the moment, and if it is about 4MV/cm which is the largest of the actually measured values, the filmthicknesses mentioned above need to be doubled. For example, about 500μm is required to obtain breakdown voltage of 100000V. To obtainbreakdown voltage of less than 600V for small home appliances, thedistance D may be smaller than 1 μm but is preferably about 1 μm atminimum in view of production stability. Thus, the distance D ispreferably not less than 1 μm and not more than 500 μm.

The donor concentration in the drift layer 11 a is one of the parametersdetermining the breakdown characteristics of the trench MOSFET 1, andgiven that breakdown field strength of Ga₂O₃ stays constant at 8 MV/cm,it is preferably not more than about 3×10¹⁷ cm⁻³ to obtain breakdownvoltage of 600V, not more than about 1.5×10¹⁷ cm⁻³ to obtain breakdownvoltage of 1200V, not more than about 5.4×10¹⁶ cm⁻³ to obtain breakdownvoltage of 3300V, not more than about 2.7×10¹⁶ cm⁻³ to obtain breakdownvoltage of 6600V, not more than about 1.5×10¹⁶ cm⁻³ to obtain breakdownvoltage of 12000V, and not more than about 2×10¹⁵ cm⁻³ to obtainbreakdown voltage of 100000V. To obtain breakdown voltage of less than600V or breakdown voltage of more than 6600V, the concentration isrespectively set to appropriate values. In addition, when the maximumbreakdown field strength of Ga₂O₃ is about 4 MV/cm, the concentrationsare respectively not more than half of the above-mentioned values.

The donor concentration and mesa width W_(m) of the channel layer 11 bare one of the parameters determining whether the trench MOSFET 1 isnormally-off or normally-on, and the lower donor concentration and thenarrower mesa width W_(m) are provided to form a normally-off device,and the higher donor concentration and the wider mesa width W_(m) areprovided to form a normally-on device. The donor concentration in thechannel layer 11 b for forming the normally-off device is, e.g., about2×10¹⁵ cm³ when having the mesa width W_(m) of 2.0 μm, about 3×10¹⁶ cm⁻³when having the mesa width W_(m) of 0.5 μm, and about 1×10¹⁷ cm⁻³ whenhaving the mesa width W_(m) of 0.2 μm. To form a normally-on device, thehigher donor concentration and the narrower mesa width W_(m) than theabove-mentioned values may be provided. The trench 16 has a depth D_(t)of, e.g., not less than 0.1 μm and not more than 5 μm.

When the width W_(m) of the mesa-shaped regions is smaller, the donorcan be added at a higher concentration and on-resistance of the channellayer 11 b can be thus more reduced. However, when the width W_(m) issmaller, there is a problem that it is more difficult to manufacture andthis causes a decrease in production yield.

For this reason, when the trenches 16 are formed by, e.g., patterningusing a general stepper, the width W_(m) of the mesa-shaped regions ispreferably not less than 0.5 μm and not more than 2 μm, and when thetrenches 16 are formed by patterning using EB (electron beam)lithography with higher resolution, the width W_(m) of the mesa-shapedregions is preferably not less than 0.1 μm and not more than 2 μm.

A width W_(t) of the trench 16 also depends on the resolution of anexposure system and is thus set within a numerical range similar to thewidth W_(m) of the mesa-shaped regions according to the type of theexposure system to be used.

The thickness of the contact layer 11 c is, e.g., not less than 10 nmand not more than 5 μm. The donor concentration in the contact layer 11c is higher than the donor concentration in the channel layer 11 b andis, e.g., not less than 1×10¹⁸ cm⁻³ and not more than 1×10²¹ cm⁻³.

The gate electrode 12 is formed of a conductor, i.e., a metal such as Nior a semiconductor containing a high concentration of a donor. The gateinsulator film 13 has, e.g., a portion 13 a covering the side and bottomsurfaces of the gate electrode 12 and electrically insulating the gateelectrode 12 from the n-type semiconductor layer 11, and a portion 13 bcovering the upper surface of the gate electrode 12 and electricallyinsulating the gate electrode 12 from the source electrode 14. Theportion 13 a and the portion 13 b of the gate insulator film 13 arerespectively formed of, e.g., HfO₂ and SiO₂. The thicknesses of theportion 13 a and the portion 13 b of the gate insulator film 13 arerespectively, e.g., not less than 10 nm and not more than 100 nm, andnot less than 50 nm and not more than 2000 nm. The breakdown-voltageperformance of the trench MOSFET 1 is more improved when the materialused to form the portion 13 a of the gate insulator film has a higherpermittivity and a larger band gap.

The n-type semiconductor layer 11 is formed of, e.g., an epitaxiallygrown film formed by the HYPE method, etc. When the n-type semiconductorlayer 11 is formed by the HYPE method, a chloride gas is used as asource material for Ga₂O₃-based single crystal or a dopant sourcematerial. Therefore, the n-type semiconductor layer 11 contains Clderived from the source material for Ga₂O₃-based single crystal or thedopant source material.

When using the HYPE method, it is possible to reduce film formation timeand the cost since the crystal growth rate is high. This feature isadvantageous particularly when forming a thick n-type semiconductorlayer 11. In addition, when using the HYPE method, it is possible toform the n-type semiconductor layer 11 with good crystal quality andthus possible to improve the production yield. Furthermore, since it ispossible to form the n-type semiconductor layer 11 with high purity, itis possible to accurately control the donor concentration.

The contact layer 11 c may be formed by implanting a donor, using an ionimplantation process, into an upper portion of the channel layer 11 bformed by epitaxial growth, but the manufacturing cost can be kept lowwhen the Ga₂O₃-based single crystal is formed by crystal growth whileintroducing a donor.

The source electrode 14 is connected to the mesa-shaped regions of then-type semiconductor layer 11 between the adjacent trenches 16. Thedrain electrode 15 is directly or indirectly connected to the n-typesemiconductor layer 11 on the opposite side to the source electrode 15.The drain electrode 15 is connected to a surface of the n-typesemiconductor substrate 10 on the opposite side to the n-typesemiconductor layer 11 in the example shown in FIG. 1, but is connectedto, e.g., a surface of the n-type semiconductor layer 11 on the oppositeside to the source electrode 14 when the trench MOSFET 1 does notinclude the n-type semiconductor substrate 10.

The source electrode 14 and the drain electrode 15 are respectivelyohmic-connected to the contact layer 11 c of the n-type semiconductorlayer 11 and to the n-type semiconductor substrate 10. The sourceelectrode 14 and the drain electrode 15 have, e.g., a Ti/Au stackedstructure.

(Modification 1)

FIG. 2 is a vertical cross-sectional view showing a modification of thetrench MOSFET 1 in the first embodiment. This trench MOSFET 1 isprovided with a p-type semiconductor member 17 which is connected to atleast some of the mesa-shaped regions of the channel layer 11 b betweenthe adjacent trenches 16 and to the source electrode 14 to provideprotection against surges.

The p-type semiconductor member 17 is used to release the surge currentcaused by a lightning strike, etc. It is possible to release electronsto the outside through the drain electrode 15 and holes to the outsidethrough the p-type semiconductor member 17 and the source electrode 14.When the p-type semiconductor member 17 is not present, it is difficultto release holes to the outside through the source electrode 14.

The p-type semiconductor member 17 is formed of a p-type semiconductorsuch as Ga₂O₃, NiO, Cu₂O, SnO, GaN, SiC, Si and GaAs, etc. The p-typesemiconductor member 17 is constantly in contact with the n-typesemiconductor layer 11 formed of a Ga₂O₃-based single crystal which isan oxide, and thus may be gradually oxidized when formed of a non-oxidesuch as Si. For this reason, the p-type semiconductor member 17 ispreferably formed of an oxide such as Ga₂O₃, NiO, Cu₂O or SnO, etc., toensure long-term stability. In addition, since it is difficult to obtainGa₂O₃ exhibiting p-type conductivity, NiO, Cu₂O and SnO, etc., areparticularly preferable as the material of the p-type semiconductormember 17.

The size, number and arrangement of the p-type semiconductor members 17are not specifically limited. When the contact area between the p-typesemiconductor member 17 and the n-type semiconductor layer 11 is larger,the surge current can be released more efficiently but it is moredifficult for the current to flow during normal operation. Therefore,the total contact area between the p-type semiconductor member 17 andthe n-type semiconductor layer 11 is preferably not less than 10% andnot more than 50% of the total contact area between the source electrode14 and the n-type semiconductor layer 11.

(Modification 2)

FIG. 3 is a vertical cross-sectional view showing another modificationof the trench MOSFET 1 in the first embodiment. In this trench MOSFET 1,an outer peripheral portion of the channel layer 11 b is removed and anouter peripheral portion of the source electrode 14 extends to theremoved region. Thus, edge portions of the source electrode 14 arelocated lateral to the outermost gate electrodes 12 at a distance fromthe outermost gate electrodes 12.

By providing the source electrode 14 having such an edge structure, itis possible to reduce concentration of the electric field at the gateelectrodes 12 closest to the edge portions of the source electrode 14and thereby further improve breakdown voltage.

Second Embodiment

The second embodiment is different from the first embodiment in that ap-type electrode is used as the gate electrode. The explanation of thesame features as the first embodiment will be omitted or simplified.

(Configuration of Trench JFET)

FIG. 4 is a vertical cross-sectional view showing a trench JFET(Junction Field Effect Transistor) 2 in the second embodiment. Thetrench JFET 2 is a vertical field-effect transistor having a trench-gatestructure.

The trench JFET 2 is provided with the n-type semiconductor substrate10, the n-type semiconductor layer 11 formed on the n-type semiconductorsubstrate 10 and having the trenches 16 opening on the upper surface (asurface opposite to the n-type semiconductor substrate 10), gateelectrodes 22 buried in the trenches 16 of the n-type semiconductorlayer 11 in a state of being prevented from contacting the sourceelectrode 14 by dielectric films 23, the source electrode 14 formed onthe upper surface of the n-type semiconductor layer 11, and the drainelectrode 15 formed on a surface of the n-type semiconductor substrate10 on the opposite side to the n-type semiconductor layer 11.

The trench JFET 2 may be either normally-off or normally-on, but isusually manufactured to be normally-off in view of safety when used as apower device. It is to prevent conduction between the source electrode14 and the drain electrode 15 in the event of power outage.

In the normally-off trench JFET 2, channels are formed in themesa-shaped regions of the n-type semiconductor layer 11 betweenadjacent the trenches 16 when voltage of not less than threshold voltageis applied between the gate electrodes 22 and the source electrode 14,allowing a current to flow from the drain electrode 15 to the sourceelectrode 14.

The sizes and materials of the n-type semiconductor substrate 10, thesource electrode 14 and the drain electrode 15 can be the same as thoseof the trench MOSFET 1 in the first embodiment. Also, the layerconfiguration, size, material and donor concentration of the n-typesemiconductor layer 11 can be the same as those of the trench MOSFET 1in the first embodiment.

The distance D from a surface of the n-type semiconductor substrate 10on the drain electrode 15 side (an interface between the n-typesemiconductor substrate 10 and the n-type semiconductor layer 11 a) tothe bottoms of the trenches 16, the depth D_(t) of the trench 16 and thewidth W_(t) of the trench 16 can be the same as those of the trenchMOSFET 1 in the first embodiment.

The gate electrodes 22 are formed of a p-type semiconductor and form p-njunctions with the n-type semiconductor layer 11. The gate electrodes 22when formed of, e.g., NiO, can exhibit p-type conductivity without beingdoped with any dopant, but may contain an acceptor impurity such as Li.

Among NiO, SnO and Cu₂O, NiO is the most preferable material for thegate electrode 22 since NiO is thermodynamically stable and allowsp-type electrodes to be stably obtained. It is difficult to form SnO dueto the existence of Sn₂O which is thermodynamically more stable thanSnO. In this regard, the conductivity of Sn₂O is unstable and it isdifficult to control to be p-type. The conductivity of Cu₂O is alsounstable and it is difficult to control to be p-type.

The p-type semiconductor constituting the gate electrode 22 preferablyincludes an amorphous portion and is more preferably configured suchthat the volume of amorphous portion is higher than the volume ofcrystalline portion. A p-type semiconductor film containing an amorphousportion can be formed at a lower temperature than an entirelycrystalline p-type semiconductor film, and thus can be manufacturedeasier at lower cost. In addition, by using the p-type semiconductorfilm containing an amorphous portion as the gate electrode 22,occurrence of leakage current can be suppressed as compared to whenusing the entirely crystalline p-type semiconductor film.

The diffusion potential at a junction between the gate electrode 22formed of the p-type semiconductor and the n-type semiconductor layer 11is often higher than the diffusion potential between the gate electrode12 and the n-type semiconductor layer 11 in the trench MOSFET 1 in thefirst embodiment.

This allows the trench JFET 2 to have a wider mesa width W_(m) than thetrench MOSFET 1, hence, it is possible to reduce the degree ofdifficulty in manufacturing without increasing conduction losses.

However, although depending on the p-type semiconductor material to beused, the trench JFET 2 has a disadvantage in that threshold voltage islow since the p-n junction is turned on when, e.g., a positive voltageof about 2 to 5V is applied to the gate electrode 22. On the other hand,the trench MOSFET 1 has an advantage in that threshold voltage can beincreased to about several V to ten and several V since the gateinsulator films are present.

The dielectric films 23 are formed of, e.g., SiO₂. The dielectric films23 have a thickness of, e.g., not less than 50 nm and not more than 2000nm.

(Modification 1)

FIG. 5 is a vertical cross-sectional view showing a modification of thetrench JFET 2 in the second embodiment. In this trench JFET 2, the outerperipheral portion of the channel layer 11 b is removed and the outerperipheral portion of the source electrode 14 extends to the removedregion. Thus, the edge portions of the source electrode 14 are locatedlateral to the outermost gate electrodes 22 at a distance from theoutermost gate electrodes 22.

By providing the source electrode 14 having such an edge structure, itis possible to reduce concentration of the electric field at the gateelectrodes 22 closest to the edge portions of the source electrode 14and thereby further improve breakdown voltage.

Third Embodiment

The third embodiment is different from the first embodiment in theshapes of the gate electrodes and the source electrode. The explanationof the same features as the first embodiment will be omitted orsimplified.

(Configuration of Trench MOSFET)

FIG. 6 is a vertical cross-sectional view showing a trench MOSFET 3 inthe third embodiment. The trench MOSFET 3 is a vertical field-effecttransistor having a trench-gate structure. The trench MOSFET 3 in thethird embodiment also includes a configuration in which gate insulatorfilms 33 (described later) are formed of a material other than oxide.

The trench MOSFET 3 is provided with the n-type semiconductor substrate10, an n-type semiconductor layer 31 formed on the n-type semiconductorsubstrate 10 and having trenches 36 opening on the upper surface (asurface opposite to the n-type semiconductor substrate 10), gateelectrodes 32 buried in the trenches 36 of the n-type semiconductorlayer 31 in a state of being covered with gate insulator films 33, asource electrode 34 formed on the upper surface of the n-typesemiconductor layer 31, and the drain electrode 15 formed on a surfaceof the n-type semiconductor substrate 10 on the opposite side to then-type semiconductor layer 31.

The trench MOSFET 3 may be either normally-off or normally-on, but isusually manufactured to be normally-off in view of safety when used as apower device. It is to prevent conduction between the source electrode34 and the drain electrode 15 in the event of power outage.

In the normally-off trench MOSFET 3, channels are formed in themesa-shaped regions of the n-type semiconductor layer 31 between theadjacent trenches 36 when voltage of not less than threshold voltage isapplied between the gate electrodes 32 and the source electrode 34,allowing a current to flow from the drain electrode 15 to the sourceelectrode 34.

In the trench MOSFET 3, the source electrode 34 is partially located inthe trenches 36, and the source electrode 34 in the trenches 36 islocated on the gate electrodes 32 via the gate insulator films 33. Inaddition, the source electrode 34 is ohmic-connected to the mesa-shapedregions of the n-type semiconductor layer 31 between the adjacenttrenches 36. The source electrode 34 has, e.g., a Ti/Au stackedstructure.

The gate insulator film 33 has, e.g., a portion 33 a electricallyinsulating the gate electrode 32 from the n-type semiconductor layer 31,and a portion 33 b covering the upper surface of the gate electrode 32and electrically insulating the gate electrode 32 from the sourceelectrode 34. The portion 33 a and the portion 33 b of the gateinsulator film 33 are respectively formed of, e.g., HfO₂ and SiO₂. Thethicknesses of the portion 33 a and the portion 33 b of the gateinsulator film 33 can be respectively equal to the thicknesses of theportion 13 a and the portion 13 b of the gate insulator film 13 in thefirst embodiment.

The gate electrode 32 is formed of, e.g., Cu, but may alternatively beformed of the same material as the gate electrode 12 in the firstembodiment.

The n-type semiconductor layer 31 has a drift layer 31 a to maintainbreakdown voltage, and a contact layer 31 b formed in the vicinity ofthe upper surface of the n-type semiconductor layer 31 by ionimplantation or epitaxial growth, etc., to provide an ohmic connectionbetween the source electrode 34 and the n-type semiconductor layer 31.The thicknesses and donor concentrations of the drift layer 31 a and thecontact layer 31 b can be respectively equal to the thicknesses anddonor concentrations of the drift layer 11 a and the contact layer 11 cin the first embodiment.

The distance D from a surface of the n-type semiconductor layer 31 onthe drain electrode 15 side (an interface between the n-typesemiconductor substrate 10 and the n-type semiconductor layer 31) to thebottoms of the trenches 36 can be equal to the distance D from a surfaceof the n-type semiconductor layer 11 on the drain electrode 15 side tothe bottoms of the trenches 16 in the first embodiment.

In addition, the depth D_(t) of the trench 36 and the width W_(t) of thetrench 36 can be respectively equal to the depth D_(t) of the trench 16and the width W_(t) of the trench 16 in the first embodiment.

The sizes and materials of the n-type semiconductor substrate 10 and thedrain electrode 15 can be the same as those of the trench MOSFET 1 inthe first embodiment.

(Effects of the Embodiments)

According to the first to third embodiments, it is possible to provide aGa₂O₃-based field-effect transistor which is formed without using ap-type β-Ga₂O₃ single crystal but is excellent in off-state leakagecharacteristics and breakdown voltage.

Example 1

Example 1 shows an example configuration of the normally-off trenchMOSFET 1 with a breakdown voltage of about 650V, where voltage when aleakage current of 1 μA flows is defined as breakdown voltage.

Firstly, an example configuration when forming the trenches 16 bypatterning using a general stepper is described. When the trenches 16are formed on the channel layer 11 b formed of a 0.5 μm-thick Ga₂O₃layer with a donor concentration of 2.0×10¹⁶ cm⁻³ so that the widthW_(m) and the width W_(t) are 0.5 μm, on-resistance of the channel layer11 b is about 0.25 mΩcm².

Meanwhile, the n-type semiconductor substrate 10, the drift layer 11 aand the contact layer 11 c, when respectively formed of a 10 μm-thickGa₂O₃ substrate with a donor concentration of 1.0×10¹⁹ cm⁻³, a 2μm-thick Ga₂O₃ layer with a donor concentration of 1.5×10¹⁷ cm⁻³, and a100 nm-thick Ga₂O₃ layer with a donor concentration of not less than1.0×10¹⁸ cm⁻³, respectively have on-resistance of not more than about0.01 mΩcm², 0.08 mΩcm² and 0.001 mΩcm².

When the trench MOSFET 1 has such a configuration, on-resistance of theentire device is about 0.34 mΩcm² and breakdown voltage is about 650V.

Next, an example configuration when forming the trenches 16 bypatterning using EB lithography with higher resolution than the generalstepper is described. When the trenches 16 are formed on the channellayer 11 b formed of a 0.5 μm-thick Ga₂O₃ layer with a donorconcentration of 1.0×10¹⁷ cm⁻³ so that the width W_(m) and the widthW_(t) are 0.2 μm, on-resistance of the channel layer 11 b is about 0.06mΩcm².

Meanwhile, the n-type semiconductor substrate 10, the drift layer 11 aand the contact layer 11 c, when respectively formed of a 10 μm-thickGa₂O₃ substrate with a donor concentration of 1.0×10¹⁹ cm⁻³, a 2μm-thick Ga₂O₃ layer with a donor concentration of 1.0×10¹⁷ cm⁻³, and a100 nm-thick Ga₂O₃ layer with a donor concentration of not less than1.0×10¹⁸ cm⁻³, respectively have on-resistance of not more than about0.01 mΩcm², 0.12 mΩcm² and 0.001 mΩcm².

When the trench MOSFET 1 has such a configuration, on-resistance of theentire device is about 0.2 mΩcm² and breakdown voltage is about 650V.

Example 2

Example 2 shows an example configuration of the normally-off trench JFET2 with a breakdown voltage of about 650V.

Firstly, an example configuration when forming the trenches 16 bypatterning using a general stepper is described. When the trenches 16are formed on the channel layer 11 b formed of a 0.5 μm-thick Ga₂O₃layer with a donor concentration of 3.0×10¹⁶ cm⁻³ so that the widthW_(m) and the width W_(t) are 0.5 μm, on-resistance of the channel layer11 b is about 0.2 mΩcm².

Meanwhile, the n-type semiconductor substrate 10, the drift layer 11 aand the contact layer 11 c, when respectively formed of a 10 μm-thickGa₂O₃ substrate with a donor concentration of 1.0×10¹⁹ cm⁻³, a 2μm-thick Ga₂O₃ layer with a donor concentration of 1.5×10¹⁷ cm⁻³, and a100 nm-thick Ga₂O₃ layer with a donor concentration of not less than1.0×10¹⁸ cm⁻³, respectively have on-resistance of not more than about0.01 mΩcm², 0.08 mΩcm² and 0.001 mΩcm².

When the trench JFET 2 has such a configuration, on-resistance of theentire device is about 0.3 mΩcm² and breakdown voltage is about 650V.

Next, an example configuration when forming the trenches 16 bypatterning using EB lithography with higher resolution than the generalstepper is described. When the trenches 16 are formed on the channellayer 11 b formed of a 0.5 μm-thick Ga₂O₃ layer with a donorconcentration of 1.5×10¹⁷ cm⁻³ so that the width W_(m) and the widthW_(t) are 0.2 μm, on-resistance of the channel layer 11 b is about 0.03mΩcm².

Meanwhile, the n-type semiconductor substrate 10, the drift layer 11 aand the contact layer 11 c, when respectively formed of a 10 μm-thickGa₂O₃ substrate with a donor concentration of 1.0×10¹⁹ cm⁻³, a 2μm-thick Ga₂O₃ layer with a donor concentration of 1.5×10¹⁷ cm⁻³, and a100 nm-thick Ga₂O₃ layer with a donor concentration of not less than1.0×10¹⁸ cm⁻³, respectively have on-resistance of not more than about0.01 mΩcm², 0.08 mΩcm² and 0.001 mΩcm².

When the trench JFET 2 has such a configuration, on-resistance of theentire device is about 0.12 mΩcm² and breakdown voltage is about 650V.

Example 3

FIG. 7 is a SEM (Scanning Electron Microscope) observation image showinga cross section of the trench MOSFET 3 in Example 3.

The trench MOSFET 3 pertaining to FIG. 7 is normally-on, and isconfigured that the n-type semiconductor substrate 10, the drift layer31 a, the contact layer 31 b, the gate electrode 32, the gate insulatorfilm 33 a, the gate insulator film 33 b, the source electrode 34 and thedrain electrode 15 are respectively formed of a 450 μm-thick Ga₂O₃substrate with a donor concentration of 6.0×10¹⁸ cm⁻³, a 5 μm-thickGa₂O₃ layer with a donor concentration of 3×10¹⁶ cm⁻³, a 2 μm-thickGa₂O₃ layer with a donor concentration of 3×10¹⁸ cm⁻³, a Cu electrodewith a thickness of 1 μm (in the vertical direction), a 50 nm-thick HfO₂film, a 300 nm-thick SiO₂ film, a 3 μm-thick Ti/Au source electrode, anda 0.3 μm-thick Ti/Au drain electrode. In addition, the trenches 36 areformed so that the depth D_(t), the width W_(m) and the width W_(t) arerespectively 4 μm, 2 μm and 4 μm.

FIGS. 8A and 8B are graphs respectively showing DC characteristics andtransfer characteristics of the trench MOSFET 3 pertaining to FIG. 7.

FIG. 8A is a graph showing a relation between voltage V_(ds) and currentdensity J_(ds) both obtained between the drain electrode 15 and thesource electrode 34, and shows curved lines obtained when voltage V_(gs)between the gate electrode 32 and the source electrode 34 is 0V, 4V, 8V,12V, 16V, 20V, 24V and 28V. The curved lines at the voltage V_(gs) of24V and 28V substantially overlap with the horizontal axis. In addition,J_(ds) is normalized by the area of the mesa top portion.

Based on FIG. 8A, it is understood that on-resistance between the drainelectrode 15 and the source electrode 34 at the voltage V_(gs) of 0V isabout 0.8 mΩcm².

Although the embodiments and Examples of the invention have beendescribed, the invention is not intended to be limited to theembodiments and Examples, and the various kinds of modifications can beimplemented without departing from the gist of the invention.

In addition, the invention according to claims is not to be limited tothe embodiments and Examples described above. Further, it should benoted that all combinations of the features described in the embodimentsand Examples are not necessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

Provided is a Ga₂O₃-based field-effect transistor which is formedwithout using a p-type β-Ga₂O₃ single crystal but is excellent inoff-state leakage characteristics and breakdown voltage.

REFERENCE SIGNS LIST

-   1 TRENCH MOSFET-   2 TRENCH JFET-   10 n-TYPE SEMICONDUCTOR SUBSTRATE-   11 n-TYPE SEMICONDUCTOR LAYER-   11 a DRIFT LAYER-   11 b CHANNEL LAYER-   11 c CONTACT LAYER-   12, 22 GATE ELECTRODE-   13 GATE INSULATOR FILM-   14 SOURCE ELECTRODE-   15 DRAIN ELECTRODE-   16 TRENCH-   17 p-TYPE SEMICONDUCTOR MEMBER

1. A field-effect transistor, comprising: an n-type semiconductor layerthat comprises a Ga₂O₃-based single crystal and a plurality of trenchesopening on one surface; a gate electrode buried in each of the pluralityof trenches; a source electrode connected to a mesa-shaped regionbetween adjacent trenches in the n-type semiconductor layer; and a drainelectrode directly or indirectly connected to the n-type semiconductorlayer on an opposite side to the source electrode.
 2. The field-effecttransistor according to claim 1, wherein the gate electrode comprises ap-type semiconductor, and the gate electrode and the n-typesemiconductor layer are in contact with each other to form a p-njunction.
 3. The field-effect transistor according to claim 1, whereinthe gate electrode comprises a conductor, and the gate electrode iselectrically insulated from the n-type semiconductor layer by a gateinsulator film.
 4. The field-effect transistor according to claim 3,further comprising: a p-type semiconductor member connected to at leasta part of the mesa-shaped region and to the source electrode.
 5. Thefield-effect transistor according to claim 1, wherein an edge portion ofthe source electrode is located lateral to outermost one of the gateelectrodes.
 6. The field-effect transistor according to claim 1, whereinthe mesa-shaped region has a width of not less than 0.1 μm and not morethan 2 μm.
 7. The field-effect transistor according to claim 1, whereina distance from a surface of the n-type semiconductor layer on a side ofthe drain electrode to a bottom of the trenches is not less than 1 μmand not more than 500 μm.